Stackage builds with new curator/Stack/Nix

[qrilka]

Stackage builds will basically work with curator build (after snapshot was generated and check with curator) - I will need to add helper scripts doing that.
The PR to Stack repo with curator updates will follow and as Stackage builds are still running expect some additional updates of build-constraints.yaml

[DanBurton]

Exciting! Is the intent to switch over entirely to building snapshots on Nix, or to do both Nix and Ubuntu builds?

[qrilka]

@DanBurton because of the way nixpkgs.haskellPackages work it looks like we couldn't easily deal with tests/benchmarks (the main problem imo is in dependency cycles) at the same time with Stack it almost just this invocation - https://github.com/commercialhaskell/stack/blob/master/subs/curator/app/Main.hs#L127 (Stack PR with some changes to make it work is to appear soon).
So at least as far as I understand the idea is to do Stackage builds/tests/bench builds with Stack and additionally test Stackage builds using Nix

[qrilka]

As commercialhaskell/stack#4582 looks to be ready this one should be also OK to merge.
With one note though - it looks like stack2nix going to be deprecates so I'm currently checking out https://github.com/input-output-hk/nix-tools for Nix builds (but it needs a bit polishing in its current state)